Fc receptors are membrane glycoproteins present on the surface of neutrophils, macrophages and other cell types, whose primary function is to bind and internalize immunoglobulins, immune complexes, and immunoglobulinopsonized viruses, bacteria, and other particles. Three distinct families of human receptors for the Fc domain of immunoglobulin G (IgG) have been identified on the basis of reactivity with monoclonal antibodies, cellular distribution, and cDNA sequences: Fc.gamma.RI, Fc.gamma.RII, and Fc.gamma.RIII. Within each of these three Fc.gamma. receptor families, distinct genes and alternative splice variants lead to a series of receptor isoforms that have striking differences in their extracellular, transmembrane, and intracellular regions. The salient features of the known classes of Fc.gamma. receptors are compiled in Table 1.
TABLE 1 __________________________________________________________________________ Fcy Receptor Families Structural Isoforms FcyRI FcyRII Fcy RIII Distinct genes A B C A B C A B __________________________________________________________________________ Splice variants + (+) a1, a2 b1, b2, b3 Allelic variants + (+) (+) HR/LR (+) NA1 NA2 Membrane anchor TM TM (TM) a1:TM TM TM y/.eta./.zeta. GPI a2:secreted complex Cell Distribution Neutrophils (+) (?) + (?) + Monocytes/M.phi. + (?) + (+) (?) + .about.5% of donors Lymphocytes B cells (?) NK cells Mesangial cells of + kidney __________________________________________________________________________
In addition to diversity based on distinct genes and their splice variants, different isoforms may also exhibit allelic polymorphisms. In several cases, the different alleles have been defined at the level of DNA sequence, and functional differences between the allelic forms have been noted. For example, the two recognized allelic forms of Fc.gamma.RIIIB, NA1 and NA2, which differ by several amino acids and N-linked glycosylation sites, also differ in their capacity to mediate phagocytosis. In the case of Fc.gamma.RIIA, the known allelic variants, HR ("high responder") and LR ("low responder"), which differ at amino acid position 131, differ substantially in their capacity to bind and internalize IgG2 (Salmon et al., 1992, J. Clin. Invest., 89:1274). (In fact, Fc.gamma.RIIA-LR is the only human Fc.gamma.R that recognizes IgG2 efficiently.) Furthermore, allelic variants of Fc.gamma.RI have also been found, though the possible functional significance of these sequence variations is not yet clear. It is likely that more than two allelic forms exist for each Fc.gamma. receptor gene.
The present inventors have identified a functionally significant allelic polymorphism in the Fc.gamma.RIIIA gene, which results in the substitution of a valine for an phenylalanine at amino acid residue number 176 in the Fc.gamma.RIIIA polypeptide (primary translation product). This amino acid substitution correlates with a differing ability of the two Fc.gamma.RIIIA variants to bind IgG. Without wishing to be bound by theory, it is believed that the distribution of the Fc.gamma.RIIIA isoform on monocytes, macrophages, mast cells, and, particularly, natural killer (NK) cells, as well as on kidney mesangial cells, means that this allelic polymorphism may affect the functioning of these cells in autoimmune diseases, tumor immunity, and antiviral and antibacterial immunity (and, in the case of mesangial cells, in mediating kidney disease). By contrast, an "expression polymorphism" was noted in one study that was interpreted as causing an apparent difference in the number (but not intrinsic binding capacity) of IgG binding sites on NK cells (Vance et al., J.Immunol. 151:6429, 1993).
Mononuclear phagocytes and NK cells lyse various target cell types via antibody-dependent cell-mediated cytotoxicity (ADCC). In ADCC, Fc.gamma.RIIIA on the cytotoxic cell recognizes antibodies coating the surface of the target cells. Binding (and possibly aggregation) of Fc.gamma.RIIIA then triggers synthesis and release of cytokines (such as, in the case of mononuclear phagocytes, interferon-.gamma., and in the case of NK cells, interleukin-2) as well as exocytic release of granules containing cytolytic compounds that effectuate lysis of the adjacent target cell. NK cells are also involved in lysis of some tumor cells and virally infected cells and participate in graft-versus-host disease (GVHD) reactions. Interestingly, occupancy of Fc.gamma.RIIIA on the surface of NK cells, coupled with exposure of the cells to interleukin-2, has also been shown to induce apoptosis (programmed cell death) in NK cells (Ortaldo et al., J. Exp.Med. 181:339, 1995; Azzoni et al., J. Immunol. 154:491, 1995).
Because of the important role played by Fc.gamma.RIIIA-bearing cells in many types of immune responses, there is a need in the art for diagnostic tests that predict the relative capabilities of Fc.gamma.RIIIA-bearing cells in an individual to participate in Fc.gamma.RIIIA-mediated reactions. Furthermore, there is a need for reagents and methods for gene therapy to supplement the Fc.gamma.RIIIA polypeptides present on an individual's immune cells with alternate allelic forms of Fc.gamma.RIIIA that are more effective in immune surveillance, immune complex clearing, and other processes.